1. Field of the Invention
The present invention relates to a dielectric filter, dielectric filter device, dielectric duplexer, and a communication device having these, which are applied to, for example, high-frequency circuits for mobile communication terminals.
2. Description of the Related Art
The mounting structure of a related dielectric duplexer is shown in FIGS. 8A to 8D. FIG. 8A is a front view of the dielectric duplexer and FIG. 8B is its bottom view. Inside a dielectric block in the form of a substantially rectangular parallelepiped, resonator holes 2a to 2f and excitation holes 3a and 3b are provided. On the outer surface of the dielectric block, an outer conductor 4 and input-output terminals 5a, 5b, and 5c are formed. The input-output terminals 5a and 5b are electrically connected to one end portion of the inner conductor formed on the inner surface of the excitation holes 3a and 3b, respectively.
FIG. 8C is a top view of a mounting substrate on which the above-mentioned dielectric duplexer is mounted and FIG. 8D is its bottom view. A broken line “A” in FIG. 8C shows an area where the dielectric duplexer is mounted. On the upper surface of the mounting substrate 6, a grounding electrode 7, input-output electrodes 8a to 8c electrically connected to the input-output terminals of the dielectric duplexer, and lines 9a to 9c extended from the input-output electrodes 8a to 8c are formed. A grounding electrode 10 is formed on the whole lower surface of the mounting substrate 6.
In such a dielectric duplexer in which a plurality of resonator holes are provided in a dielectric block, each resonator operates in the TEM mode. However, in the dielectric block in the form of a substantially rectangular parallelepiped on the outer surface of which the outer conductor 4 is formed, the space enclosed by the outer conductor 4 functions as a resonance space for the TE mode and spurious signals (unwanted waves) of the TE mode are generated. This becomes a problem since, in particular, the TE101 mode (mode represented by TExyz when the length and breadth of the mounting surface of the dielectric block are x and y axes and the height is the z axis) becomes close to the frequency band of the TEM mode to be used.
In communication equipment such as portable communication terminals (portable telephones), etc., in designing the devices, a large attenuation value in a frequency band that is two or three times as wide as the transmission frequency band must be secured. For example, in the case of a W-CDMA portable telephone system, since the transmission frequency band on the terminal side is 1920 to 1980 MHz, twice the frequency band (3840 to 3960 MHz) or three times the frequency band (5760 to 5940 MHz) must be secured. In a dielectric filter using the related dielectric block, when a duplexer for the W-CDMA terminal is designed, the peak frequency of the TE101 mode is in the vicinity of 4000 MHz, which is close to twice the transmission frequency band.
Although the peak frequency of the TE101 mode can be changed by the outer dimensions of the dielectric block, when the shift of the peak frequency of the TE101 mode to a fixed frequency is added as a design element, it is not possible to obtain the best characteristics of the TEM mode to be used.
What is described above is valid not only for dielectric duplexers, but also for dielectric filters using dielectric blocks.